Depends on the extent of the cracks. If there are lots of cracks, everywhere, then for christ's sake get it skimmed by a plasterer.
For smaller cracks, it's most effective to, ironically, chiv out a gully along the line of the crack, just a small one, to make filling it look better overall. It's hard to fill a hairline crack, it's easier to fill a slightly deeper gully. You fill it with polyfiller (don't buy a tub, buy a box of power and mix it, waaaaay cheaper), wait for it to dry, then sand it down and paint. For small cracks this will look totally fine.
Now, don't do what I tried to do, and use this method for a wall FULL of cracks. I tried to paint it and it looked shit, and took forever.